To perform efficient communication through a network such as the Internet, it is important for a sender to comprehend how much traffic the network can handle and to perform transmission within the capacity of the network. Nonetheless, no method enabling one to directly know this exists and therefore, various measurement techniques have been used (for example, refer to Japanese Patent Nos. 4853319 and 4367505; Japanese Laid-Open Patent Publication Nos. 2006-074773 and 2006-340081; S. Floyd, T. Henderson, A. Gurtov, “The NewReno Modification to TCP's Fast Recovery Algorithm”, RFC2582, Internet Engineering Task Force (IETF), 2003; and Lawrence S. Brakmo, Larry L. Peterson, “TCP Vegas: End to End Congestion Avoidance on a Global Internet”, IEEE Journal on Selected Areas in Communications, Vol. 13, No. 8, pp. 1465-1480, 1995.
For example, Japanese Patent No. 4853319 discloses a technique in which a recipient observes a reception rate and notifies the sender of the reception rate. However, if such transmission band control is introduced into an existing communication procedure, a function for measurement is also added to a communication device on the receiving side. Japanese Patent No. 4367505 discloses that the sender estimates the reception rate for a recipient by measuring the pace of arrival of acknowledgment returned by the recipient to the sender or a round trip time (RTT) thereof. With this method, the capacity of the network can be estimated solely from information acquired by the sender. However, this measurement method has a problem in that the method is easily affected by the state of the network in the return path through which the acknowledgment is returned. In another conventional technique, feed-back control is used such that the transmission rate is gradually increased while monitoring for signs of the transmission rate having exceeded or exceeding the ability of the network such as packet loss and an increase in propagation delay (Japanese Laid-Open Patent Publication Nos. 2006-074773 and 2006-340081; S. Floyd, T. Henderson, A. Gurtov, “The NewReno Modification to TCP's Fast Recovery Algorithm,” RFC2582, Internet Engineering Task Force (IFTF), 2003; and Lawrence S. Brakmo, Larry L. Peterson, “TCP Vegas: End to End Congestion Avoidance on a Global Internet,” IEEE Journal on Selected Areas in Communications, Vol. 13, No. 8, pp. 1465-1480, 1995). Nonetheless, this technique has a problem in that the technique inherently requires time for convergence and is unable to deal with dynamic fluctuations in the network.